How long should the Blender BMW demo file take to render?
It depends on your hardware. The BMW &quot;blenchmark&quot; should take however long it takes to render without you modifying the file.
The idea of such benchmark is for folks to see how their particular hardware compares to other in similar circumstances, with the same scene with the exact same settings.
The results are here:
http://blenchmark.com/gpu-benchmarks

Are there ways to speed it up? yes, but that is not the point of said file.
If everybody modifies the file, then there will be way too many variables and the statistics and comparisons for rendering are no longer valid.